1. Field of the Invention
The present invention relates to a multiple input-multiple output (MIMO) system and a method of user scheduling, especially to an adaptive MIMO system and an adaptive user scheduling method.
2. Description of the Related Art
The future wireless communication system is required to support the extremely high speed data traffics, such as the videoconference, the video-on-demand and the interactive video game, etc. As required in the ITU-R M1645, it should support up to 100 Mbps for high mobility traffics and up to 1 Gbps for low mobility or fixed wireless traffics. The data rate of one wireless channel equals to the product of its spectrum width and the spectrum efficiency of the adopted technology. In order to improve the data rate, the spectrum width of the channel and the spectrum efficiency of the adopted technology should be improved. However, since the frequency resource is limited, the communication speed cannot be raised by infinitely increasing the spectrum width. Improving the spectrum efficiency of the adopted technology is one optimal solution for resolving the problem. Recent reseach has discovered that the MIMO technology can be used to improve the spectrum efficiency.
The so-called MIMO technology means that mutilple antennas are mounted at both the transmitting terminal and the receiving terminal in a communication system. The MIMO technology also includes that multiple antennas are mounted at either side, i.e., the single input-multiple output (SIMO) and the multiple input-single output (MISO). Different antennas are physically separated, and are generally regarded as introducing an additional signal domain—a space domain, into the communication system.
In the MIMO system, two signal processing methods are generally adopted to improve the spectrum efficiency of channels.
The first method is called the space-time coding (STC). It converts one original data stream into nT code streams by an encoder group and transmits them from different antennas (e.g., nT antennas) respectively. Each code stream is a different version of the original data stream and has the correlation with itself in time domain, and the correlation also exists between code streams. Thus, a better BER performance will be obtained by using these correlations at the receiving terminal having nR antennas, or, the spectrum efficiency will be improved by increasing the number of bits on each signal symbol when the BER performance is kept constantly. The gain that is obtained in the space domain by the space-time coding is called the diversity gain and the diversity gain provided by the MIMO system is nT×nR.
The second signal processing method is called the layered space-time signal processing (LAST). It divides an original data stream into nT independent code streams and transmits each code stream from a different antenna, respectively. Each code stream is a part of the original data stream and no correlation exists between these code streams. At the receiving terminal, each code stream transmitted from different antennas is decomposed by multimensional signal processing methods, such as, the maximum likelihood (ML), the minimum means square error (MMSE), or the zero-forcing (ZF). Thus, nT independent channels are established between the receiving terminals and the transmitting terminals and the frequency efficiency is increased by about nT times. A gain that is obtained at the space domain by the layered space-time signal processing is called the multiplexing gain. A MIMO system can provide a maximum multiplexing gain which equals to the minimum one of the numerals, nT, i.e., min (nT, nR).
Research has discovered that in a single user point-to-point MIMO communication system, there is a tradeoff between the diversity gain and the multiplexing gain provided by the space domain: the more the diversity gain, the less the multiplexing gain, and vice versa.
However, the modern communication systems are constructed based at least in part on a cellular structure, and the basic communication model thereof is that one base station in the cellular serves a plurality of users simultaneously, which leads to a problem of the multiple access. Traditional accessing methods include FDMA, TDMA and CDMA, which are all based on the circuit switch principle, i.e., each user is assigned with a fixed frequency width (for FDMA), a fixed time slot (for TDMA) or a fixed spread code (for CDMA).
In GSM, for example, the base station assigns eight time slots of a frame to eight users in the manner of the fixed time slot assignment on a 200K channel. The method can ensure the time delay characteristic of communication traffics and fits the traffics sensitive to the time delay, such as the voice communication. But the disadvantage of the method is that the resource assignment is set regardless of the conditions of the wireless channels. However, conditions of wireless channels change greatly, the system will lose its performance if users are assigned with the channels that are just in a deep fading.
The future communication system will mostly focus on data traffic and be not strict with the time delay. Then, the packet switch is acceptable for the multiple access. When conducting the packet switch, the base station is required to assign channels to different users in real time, which is called the user scheduling. Two basic user-scheduling methods are being used currently in the wireless communication system. One is the Round Robin scheduling, in which channels are assigned to all users in a manner of the round robin. Similar to the circuit switch, the method can ensure the time delay characteristic and the fairness for users but cannot improve the performance of the system. The other is the Maximum C/I scheduling. It can assign channels to users having the maximum C/I according to current channel-fading conditions, thereby improving the system performance greatly. The gain that is obtained by the Maximum C/I scheduling is called the multiuser diversity.
Research also indicates that in the conventional multiuser single input single output system (MuSISO system), the system performance can reach the maximum by assigning channels to users having the maximum C/I. But the result cannot be applied to the multiuser multi-antenna system—the multiuser MIMO system. While applied to a multiuser system, multiple antennas can provide not only the multiuser access—the spatial division multiple accesses (SDMA), but also the diversity gain and multiplexing gain. By using the spatial division multiple access, a user permitted to be accessed is assigned with a certain spatial resource to create corresponding independent communication links, and the spatial resource of each user can be used to provide the diversity gain or the multiplexing gain. Research further shows that in case of multiple antennas, the system performance can reach the maximum only when channels are simultaneously assigned to one or more users. The above discovery, however, is only a guidance for a theory and lacks an efficient optimal user scheduling method.
For downlink of communication system, the spatial division multiple access can be performed by using methods of transmitting signal processings, such as the dirty paper coding (DPC) and the transmit beamforming (TBF), at base stations. But the method requires that transmitting terminals (base stations) know the precise fading coefficient of downward channels, which, however, is difficult to be realized in an actual system. Another method for performing the spatial division multiple accesses is by using the receiving signal processing. Concretely, the useful information is processed by using the method of the space coding or layered space-time signal processing at transmitting terminals and demodulated by interference elimination or signal detection at user terminals. Since the space-time coding and layered signal processing do not require the fading coefficient of the downward channels and are therefore suitable for performing the spatial division multiple access of the downward channels.
Further, when using the space-time coding based multiuser system, the performance of multiuser scheduling systems is poorer than that of the single antenna system. Therefore, in the multiuser scheduling system, it is apt to adopt the layered space-time signal processing based multiple input-multiple output system for each user permited to access, i.e., the transmitting terminals find out a users group according to the limited channel feedback information and assign antennas to all users of the user group in order to transmit an independent code stream of each user from each antenna assigned to the user. When the number of receving antennas nR is larger than that of transmitting antennas nT at the transmitting terminals, each user can establish an independent interference-free channel for each transmitting antenna. And in such case, the assignment of each transmitting terminal does not interfere the assignment of other antennas. U.S. Pat. No. 6,662,024 discloses a user scheduling arithmetic of the multiuser multiple input-multiple output sytem at the precondition of nR≧nT. But when the number of receiving antennas is smaller than that the number of transmitting antennas, an independent interference-free channel cannot be established for each transmitting antenna according to the method disclosed in the patent and the method disclosed in U.S. Pat. No. 6,662,024 thus cannot be used.
Reference 1 (D. J. Mazzarese and W. A. Krzymien, [2003], “High throughput downlink cellular packet data access with multiple antennas and multiuser diversity”) discloses a user scheduling method when nR=1. It contends that the number of the scheduled users is always nT, so that all nT transmitting antennas can be assigned to nT users, respectively. However, the problem is that the optimal performance of the system cannot be ensured.
Reference 2 (D. Aktas and H. E. Gamal, [2003], “Multiuser scheduling for MIMO wireless systems”) deems that the number of the scheduled users should be a predetermined number L (1≦L≦nT) which requires to select L antennas from all nT transmitting antennas and assigns them to selected L users, respectively. The method is only efficient when the number L has been given since the value of L is not clear yet.
Methods disclosed in References 1 and 2 have following defects:                1) They are the methods of scheduling fixed number of users and the number of scheduled users is supposed to be known;        2) Their arithmetics cannot ensure the scheduling of all the supposed scheduled users, which leads to a loss of the performance of the system;        3) Solutions are all given when nR=1, and no concrete arithmetic is given when nR>1 due to the high complexity of the arithmetics.        
Therefore, those disclosed methods cannot provide the optimal user scheduling according to channel conditions, i.e., they cannot provide the maximum system capacity.